Polyarylene sulfide resins, exemplified by PPS, are known for their chemical and heat resistance. For instance, a PPS nonwoven fabric can be used for extended periods at temperatures as high as 190.degree.C. This is in contrast to typical maximum continuous use temperatures of 80.degree. C. for polypropylene fiber and 120.degree. C. to 130.degree. C. for polyester fiber. PPS fiber also has superior wet heat resistance, e.g., it can be used in steam having a temperature of up to 160.degree. C. As a result of its excellent heat resistance, wet heat resistance, resistance to chemicals, and fire retardant properties, polyarylene sulfide has been used in a wide variety of products, such as filters, fire retardant electrical insulating materials, battery separators, and separators for electrolyzers used in producing hydrogen and oxygen by water electrolysis.
The preparation of fibers and fibrous articles from arylene sulfide polymers, particularly PPS, has been known for some time. In general, a polyallylene sulfide polymer such as PPS is melted in an extruder. The molten polymer is then extruded from a plurality of orifices. A heated high pressure gas is fed on both sides of each orifice to blow a stream of molten polymer. The molten polymer is drawn, thinned, and set to a shape of fine fiber. The fiber is then collected as a web on a screen collector.
There has been some suggestion that PPS fibers can be prepared by heating PPS polymer, which has an inherent viscosity of 0.15 to 0.25, at an elevated temperature for a period of time sufficient to at least partially cure the polymer and thus produce a polymer with a melt flow rate or index in the range of 75 to 800. The partially cured polymer is then melt-spun through orifices, followed by drawing of the thus melt-spun filaments while still in the molten state.
Unfortunately, many difficulties have been encountered in the melt blowing of polyarylene sulfide resins, particularly of PPS resins. Melt blowing represents the most difficult melt extrusion process as a result of the elevated temperatures which are employed in melt blowing, which are generally up to about 40-70.degree. C. higher than other melt processes, and the concomitant polymerization of polyarylene sulfide and decomposition and formation of char. During the melt blowing of such fibers, which involves heating the polyarylene sulfide resin and passing the molten resin through suitable fiberizing nozzles, the molten resin has a tendency to polymerize and/or crosslink, thereby resulting in spitting, formation of shots or balls, and clogging of the fiberizing nozzles, as well as generally restricting the fineness and quality of the formed fibers.
Attempts have been made in the industry to overcome some of these difficulties. For example, molten PPS polymer has been forced through a primary filter means having a maximum absolute pore rating of no more than about 125 .mu.m to provide a molten primary filtered polymer, and then the molten primary filtered polymer has been forced through a secondary filter means having a maximum absolute pore rating of no more than 80 .mu.m to provide a secondary filtered polymer. Reportedly, such a method allows for the preparation of a nonwoven fabric composed of fine PPS fibers having substantially no polymer balls. The method employs a PPS having a linear high polymer structure which has high thermal stability in the molten state and a good flowability as compared to the crosslinked-type polyallylene sulfide.
In another attempt to overcome some of the aforesaid difficulties, the melt blowing equipment is frequently started up and shut down and/or otherwise purged on a regular basis with another polymer, usually with a polyolefin such as polypropylene. Such procedures, however, only temporarily alleviate the aforesaid difficulties and impair the efficiency of fiber production, as well as the quality and quantity of fibers produced.
Thus, there exists a need for an improved method of melt blowing polyarylene sulfides which avoids, at least in part, the aforesaid difficulties, such as shot formation and clogging of the melt blowing apparatus, while being relatively easy to practice.
The present invention seeks to provide such a method of melt blowing polyarylene sulfides, as well as the resulting polyarylene sulfide fibers and other products. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.